The present invention relates to ophthalmic testing instruments, and more particularly, to apparatus and methods for testing the binocular vision of a patient, as well as for testing monocular vision during binocular viewing.
The determination of visual acuity is an essential part of every eye examination. During the course of such an examination, acuity may be measured repeatedly to ascertain the resolution of each eye independently and both eyes together. The determination of binocular function and motor function is an essential part of the process of refracting and determining the optimal corrective lenses as well as providing a means for assessing the progress of ocular pathology.
Originally, clinical methods for measuring visual acuity and binocular function involved the use of wall charts containing a fixed array of Snellen letters, "Tumbling E" targets, and other accepted acuity images and characters or symbols. The patient ordinarily viewed the charts from a fixed distance (usually 20 feet). With the advent of ophthalmic devices, clinical testing methods have become more sophisticated. Electromechanical devices were developed, including the American Optical Project-O-Chart manually operated glass slide projector, and later, a remote controlled glass slide projector. Both types of devices suffered from the inability to present more than a few different visual acuity targets at any given target size. U.S. Pat. No. 4,239,351 solved this problem by disclosing a completely digital electronic apparatus for generating and displaying symbols to be used as targets for testing visual acuity.
The present invention relates to a visual function tester for testing binocular vision, ocular motor imbalance, such as phoria, associated phoria and fixation disparity, and refining binocular refractions. The accurate diagnosis of several ophthalmic disorders requires a binocular testing environment that allows some images to be made visible to one eye and invisible to the other, while some characters may be visible to both eyes. Targets also are generated such that they appear to the patient to be out of the plane of the screen (i.e., they appear to be closer or farther away than the other targets). Such an environment is critical to the diagnosis and treatment of such disorders as, for example, monofixation, fixation disparity, amblyopia, convergence axis, divergence axis, convergence insufficiency, and for detecting malingering.
Previous efforts to provide a test environment closely approximating the normal binocular situation have not proven successful. For example, the four prism-diopter base-out test has been used to determine the existence of bifixation (central fusion) and monofixation (absence of central fusion). While the patient reads letters at a distance of six meters, a four diopter base-out prism is slipped first before one eye and then the other. The prism covered eye is watched closely for movement. Absence of movement by one of the eyes identifies a monocular scotoma in that eye. Bifixation is identified by each eye moving inward to refixate in response to the image displacement produced by the prism. (See Clinical Ophthalmology, by Thomas D. Duane, M.D., Ph.D., Vol. 1, Chap. 9, pp. 8 & 10.)
The four diopter base-out prism test has not proven reliable, however, because, occasionally, bifixating patients recognize diplopia when the prism is slipped before either eye, but make no attempt to restore bifixation by convergence. Also, many orthophoric monofixating patients who have good acuity in each eye rapidly alternate their fixation to the uncovered eye as the prism is slipped before the fixating eye; consequently, neither eye shows a movement response.
A second method used to approximate a binocular testing environment employs the A-O Vectographic Project-O-Chart slide (originally manufactured by the American Optical Company). A high resolution, high contrast vectograph printing process is used to produce character slides. Each character on the slide has a self-contained light polarization. When a pair of these polarized characters with axes of polarization 90 degrees to each other are superimposed, each will function independently without optical interference from the other. When the resulting single slide is projected on a non-depolarizing screen and viewed through "analyzers," or polarized glasses, some images are made visible to one eye and invisible to the other. Some portions of the slide also contain characters that are seen by both eyes.
Although the Project-O-Chart slide does provide a more rapid and dependable differentiation than the four diopter base-out prism test, it has not proven successful and has not gained industry acceptance. Production of the character slides and compatible analyzers requires an extremely tight registration of polarization, which is difficult to produce with the high degree of accuracy required for effective operation of the test and which is sensitive to head position. It also suffers from the same deficiencies presented generally by the Project-O-Chart method. There is no flexiblity in the chart printed--the characters on the slide are permanent. As the patient is tested and re-tested, he or she begins to memorize the test characters. Furthermore, dirt on the slide, readjustment of the focus of the projector, and dimming light bulbs are all problems inherent in the Project-O-Chart.
In view of the foregoing, it is an object of this invention to provide improved methods and apparatus for testing binocular vision.
It is a more particular object of this invention to provide methods and apparatus for testing binocular vision that allow an eye examiner quickly and accurately to produce a wide variety of visual targets, including targets which appear to be in front of or behind the surface of the monitor.